Throughout the world, there are many situations where the results of industrial and mining activities create large volumes of waste waters which contain high concentrations of sulphates. Most of these waste waters also contain substantial concentrations of dissolved metals, typically including iron, manganese, lead, zinc, copper, arsenic and nickel. These waste waters frequently have a low pH.
Many of these waste waters cause substantial environmental problems and, in a number of cases, the entity which created the problem is no longer in existence or is unable to carry the costs associated with existing technologies for managing the waste water in a satisfactory manner.
There are a large number of technologies available for the treatment of these waste waters and for recovering the water to a quality which is fit for re-use at the site that produced the waste water, for use by others or for safe discharge into the environment. However, all of these technologies come with an associated cost. In addition, these technologies create by-products and process residues. These by-products and process residues are derived from the contaminants which have to be removed from the waste water as well as from components within the chemical reagents which have to be used within the particular treatment process that is being applied. Frequently, the disposal costs associated with these by-products and process residues greatly exceed the purchase costs of the necessary treatment reagents.
In most instances, these by-products and process residues contain components which would have a marketable value (often a substantial value) if it were not for the other components that are frequently also present within the bi-product or process residue. These other components are typically derived from the waste water or they arrive as contaminants during the course of treatment. There is therefore a major need for low cost technologies capable of treating and recovering these waste waters which, ideally, would also create residues with a marketable value rather than a disposal cost.
Most of the currently applied technologies seek to remove the sulphate content from the waste water in the form of gypsum. Gypsum has a low but significant solubility. This means that following the precipitation of the gypsum there is a residual sulphate concentration within the water; typically between about 3 and 10 times the maximum acceptable concentration for discharge to the environment or for drinking water purposes.
The normal means that are applied to the waste water in order to create an acceptably low sulphate concentration include membrane-based processes: reverse osmosis, nano-filtration or a dialysis based process. These membrane-based processes have both a high capital cost and a high operating cost. In addition, they frequently suffer from a number of blinding and fouling mechanisms which can result in frequent shut downs for cleaning and to a short operating life for the membranes themselves.
An alternative technology is to exploit the very much lower solubility of ettringite. Ettringite has a complex crystal structure. This structure is able to include many other ionic components within the crystal lattice, both anions and cations. Ettringite has the generally accepted formula of: Ca6Al2(SO4)3(OH)12.26H2O.
For most multi-valent metals, the pH at which the solubility of the hydroxide is at a minimum is specific to the respective metal. Also, the rate of change within the solubility of each of these hydroxides as the pH is lowered or raised is also slightly different for each of the metals, especially at pH levels which are above the pH of their lowest solubility. This means that as the pH is raised within an acidic solution that contains a number of different metals, the metal with the lowest pH for its minimum solubility tends to be the first to be precipitated from the mixture, followed by the metal with the next lowest pH for its minimum solubility and so on as the pH is raised.
The initial concentrations of the respective metals within the solution will also influence the pH at which each metal hydroxide begins to precipitate. For most industrial or mining derived waste waters, iron is typically present in large quantities. Iron has two principal oxidation states, ferrous Fe (II) and ferric Fe (III). Ferric iron is normally at its lowest solubility at a pH of between pH 3.5 and 4.0, whereas the normal minimum solubility for ferrous iron is in the region of pH 9.5. The equivalent pH for aluminium is between pH 6 and 7, whereas for most of the other transition metals, the normal minimum solubility pH occurs between pH 8 and pH 10. Manganese is the typical exception for the metals which are normally present at relatively high concentration within many of the waste waters that are derived from heavy industry and from mining activities. It has a pH for its minimum solubility of around pH 10.5 to pH 11. The equivalent pH for both silver and cadmium is in the region of pH 12.
Given the above, it can be readily appreciated that, as the pH of a typical metals containing waste water is raised, most of the Fe (III) will be precipitated before any substantial amounts of aluminium are precipitated. Additionally, most of the aluminium will be precipitated before any substantial amounts of Fe (II) or any of the other multi-valent metals are precipitated.
Similarly, by the time the pH has been raised to that which is necessary for the practical precipitation of ettringite virtually all of the likely multivalent metals within the solution will have been precipitated.
At a high pH (above its minimum solubility), aluminium exists in solution predominantly as the hydrated negative ion Al(OH)4−. At low pH (below its minimum solubility), it exists as a positive ion. The degree of hydration of the positive ion varies from Al3+ at low pH through Al(OH)2+ to Al(OH)2+ as the pH rises towards the minimum solubility. As the pH is lowered, so the solubility of the various aluminium species increases. Also, as the pH is lowered, the number of hydrogen ions that are used in order to create the higher charge on each aluminium ion will increase. The net result of this behaviour is that in order to form ettringite, four OH− ions are needed per molecule of ettringite, together with two Al(OH)4− ions, 3 sulphate ions and 6 calcium ions. However, when an ettringite molecule is dissolved at low pH, it can release up to 12 hydroxide ions, depending on the pH linked degree of hydration of the aluminium ions. This means that per unit of calcium, the neutralising capability of ettringite under low pH conditions is the same as the neutralising capability of slaked lime.
Ettringite crystals require a high pH and sufficient aluminium, as well as the necessary calcium and sulphate in order for them to grow. As a result of the typically acidic nature and the high metals content of the waste waters that are often associated with former mine workings, a substantial quantity of neutralising medium has to be added before ettringite can be created. Lime, because of its relatively low price and general availability, is frequently used to supply this neutralising function. Additionally, this input of lime is normally able to provide the necessary calcium input for an effective ettringite based treatment process.
Usually, if the sulphate content of the waste water is high enough, gypsum is precipitated within a first stage of the treatment process. Preferably, the precipitated solids are then removed before the water is routed to a second treatment stage. Within this second treatment stage, a water soluble aluminium reagent is usually added, together with more lime. An example of this treatment process is the CESR Process where the soluble aluminium reagent is a proprietary reagent powder. The process was developed within Eastern Europe and has been widely applied to mining related waste waters within Europe.
In most situations the aluminium reagent has a prohibitively high price. This has led to a number of developments whereby most of the aluminium is recovered from the ettringite product and re-used within this second treatment stage. Depending on the specifics of the particular aluminium recovery process that is applied, it is usual for a large amount of additional gypsum residue to be created.
Once the ettringite that has been produced by the treatment process has been removed from the water (usually by a combination of gravity settlement and filtration) the ettringite can be re-dissolved within a lower pH environment. Some of the existing technologies use sulphuric acid, for example, within the SAVMIN Process, to create and maintain this lower pH environment. Others, for example, Veolia in South America, use hydrochloric acid, or a mixture of sulphuric and hydrochloric acid.
With careful control, the pH can be maintained at a level which is low enough for the ettringite to dissolve but high enough so that the aluminium that is released from the ettringite is immediately precipitated in the form of amorphous aluminium hydroxide. At the same time, the calcium and the sulphate portion of the ettringite is normally precipitated in the form of gypsum. Finally, the mixture has to be separated into a gypsum product or residue and a suitably pure aluminium hydroxide for return to the sulphate removal stage within the overall process. For economic reasons, the gypsum must contain as little aluminium as possible.
Within the SAVMIN Process, the slow rate of crystallisation of gypsum is exploited. The aluminium hydroxide can be made to precipitate rapidly and providing the resultant precipitate is removed rapidly from the reaction mixture, there is relatively little gypsum contamination within the aluminium hydroxide. The gypsum is then precipitated within a subsequent stage, where the kinetics of precipitation are normally assisted by a gypsum seeding process using either fresh or recycled gypsum.
Within the Veolia Process, the addition of the hydrochloric acid to the separated ettringite creates a strong solution of calcium chloride. Calcium chloride is an extremely soluble salt and a very high ionic strength solution can be created. Under these conditions it is possible to increase the solubility of gypsum to the extent that with appropriate control over the water content of the mixture, only the aluminium hydroxide is precipitated and the sulphate that is released remains in solution as the ettringite is dissolved. This process unfortunately creates a concentrated brine residue which requires disposal.
An alternative approach is to utilise a substantially lower pH for the dissolution of the ettringite. With this approach, the aluminium remains in solution and the gypsum can be separated as a high purity product by simple gravity separation, filtration or by other appropriate means. One down side of this option is the cost of the extra acid to carry out this dissolution. Additionally, extra lime is needed within the ettringite production stage in order to raise the pH of the recovered aluminium solution to the high pH that is needed for ettringite crystallisation. Further, if hydrochloric acid or another monovalent acid is used for this pH reduction, either a concentrated brine is created (for example, if the aluminium is precipitated as the hydroxide so as to enable only the aluminium hydroxide to be returned to the ettringite precipitation stage), or a high concentration of chloride or other monovalent anion is introduced into the product water.
Unfortunately, the reagent costs associated with these aluminium recovery options, plus the inevitable aluminium losses which occur within them, have meant that the currently practiced ettringite-based processes represent an expensive method for reducing the sulphate concentration.
Providing the ionic strength of the solution is not too high, gypsum precipitation is normally able to achieve a sulphate concentration in the order of 1500 to 3000 mg/liter. This is considerably above the 100 to 25 mg/liter of sulphate that is required for many of the options for either the re-use or the discharge of the treated water. This, combined with the high operating costs of the currently practiced ettringite based processes, have led to a general preference by water treatment specialists for the use of membrane-based approaches rather than ettringite for this sulphate reduction step.
The present invention proposes a recycling method for the aluminium content of ettringite which does not require the use of an acid addition step. Instead, it exploits the typically acidic nature of the waste water, along with the typical iron (and other multi-valent metal) content of the waste water that is used as the feed water for the process.